Congenital heart defects and 22q11 deletions: which genes count?

Identification of candidate genes for congenital heart defects on proximal chromosome 8p

With the application of advanced molecular cytogenetic techniques, the number of patients identified as having abnormal chromosome 8p has increased progressively. Individuals with terminal 8p deletion have been extensively described in previous studies. The manifestations usually include cardiac anomalies, developmental delay/mental retardation, craniofacial abnormalities, and multiple other minor anomalies. However, some patients with proximal deletion also presented with similar phenotypic features. Here we describe a female child with an 18.5-Mb deletion at 8p11.23–p22 that include the cardiac-associated loci NKX2-6 and NRG1. Further mutation screening of these two candidate genes in 143 atrial septal defect patients, two heterozygous mutations NKX2-6 (c.1A > T) and NRG1 (c.1652G > A) were identified. The mutations were described for the first time in patients with congenital heart disease (CHD). The c.1A > T NKX2-6 generated a protein truncated by 45 amino acids with a decreased level of mRNA expression, whereas the NRG1 mutation had no significant effect on protein functions. Our findings suggest that 8p21-8p12 may be another critical region for 8p-associated CHD, and some cardiac malformations might be due to NKX2-6 haploinsufficiency. This study also links the NKX2-6 mutation to ASD for the first time, providing novel insight into the molecular underpinning of this common form of CHD.

A defect in early myogenesis causes Otitis media in two mouse models of 22q11.2 Deletion Syndrome

Otitis media (OM), the inflammation of the middle ear, is the most common disease and cause for surgery in infants worldwide. Chronic Otitis media with effusion (OME) often leads to conductive hearing loss and is a common feature of a number of craniofacial syndromes, such as 22q11.2 Deletion Syndrome (22q11.2DS). OM is more common in children because the more horizontal position of the Eustachian tube (ET) in infants limits or delays clearance of middle ear effusions. Some mouse models with OM have shown alterations in the morphology and angle of the ET. Here, we present a novel mechanism in which OM is caused not by a defect in the ET itself but in the muscles that control its function. Our results show that in two mouse models of 22q11.2DS (Df1/+ and Tbx1(+/-)) presenting with bi- or unilateral OME, the fourth pharyngeal arch-derived levator veli palatini muscles were hypoplastic, which was associated with an earlier altered pattern of MyoD expression. Importantly, in mice with unilateral OME, the side with the inflammation was associated with significantly smaller muscles than the contralateral unaffected ear. Functional tests examining ET patency confirmed a reduced clearing ability in the heterozygous mice. Our findings are also of clinical relevance as targeting hypoplastic muscles might present a novel preventative measure for reducing the high rates of OM in 22q11.2DS patients.

Hearing loss in a mouse model of 22q11.2 Deletion Syndrome

22q11.2 Deletion Syndrome (22q11DS) arises from an interstitial chromosomal microdeletion encompassing at least 30 genes. This disorder is one of the most significant known cytogenetic risk factors for schizophrenia, and can also cause heart abnormalities, cognitive deficits, hearing difficulties, and a variety of other medical problems. The Df1/+ hemizygous knockout mouse, a model for human 22q11DS, recapitulates many of the deficits observed in the human syndrome including heart defects, impaired memory, and abnormal auditory sensorimotor gating. Here we show that Df1/+ mice, like human 22q11DS patients, have substantial rates of hearing loss arising from chronic middle ear infection. Auditory brainstem response (ABR) measurements revealed significant elevation of click-response thresholds in 48% of Df1/+ mice, often in only one ear. Anatomical and histological analysis of the middle ear demonstrated no gross structural abnormalities, but frequent signs of otitis media (OM, chronic inflammation of the middle ear), including excessive effusion and thickened mucosa. In mice for which both in vivo ABR thresholds and post mortem middle-ear histology were obtained, the severity of signs of OM correlated directly with the level of hearing impairment. These results suggest that abnormal auditory sensorimotor gating previously reported in mouse models of 22q11DS could arise from abnormalities in auditory processing. Furthermore, the findings indicate that Df1/+ mice are an excellent model for increased risk of OM in human 22q11DS patients. Given the frequently monaural nature of OM in Df1/+ mice, these animals could also be a powerful tool for investigating the interplay between genetic and environmental causes of OM.

Inactivation of Bmp4 from the Tbx1 expression domain causes abnormal pharyngeal arch artery and cardiac outflow tract remodeling

Maldevelopment of outflow tract and aortic arch arteries is among the most common forms of human congenital heart diseases. Both Bmp4 and Tbx1 are known to play critical roles during cardiovascular development. Expression of these two genes partially overlaps in pharyngeal arch areas in mouse embryos. In this study, we applied a conditional gene inactivation approach to test the hypothesis that Bmp4 expressed from the Tbx1 expression domain plays a critical role for normal development of outflow tract and pharyngeal arch arteries. We showed that inactivation of Bmp4 from Tbx1-expressing cells leads to the spectrum of deformities resembling the cardiovascular defects observed in human DiGeorge syndrome patients. Inactivation of Bmp4 from the Tbx1 expression domain did not cause patterning defects, but affected remodeling of outflow tract and pharyngeal arch arteries. Our further examination revealed that Bmp4 is required for normal recruitment/differentiation of smooth muscle cells surrounding the PAA4 and survival of outflow tract cushion mesenchymal cells.

Prenatal diagnosis of membranous ventricular septal aneurysms and their association with absence of atrioventricular valve 'offsetting'

Congenital aneurysm of the membranous portion of the ventricular septum in association with absence of atrioventricular valve 'offsetting' was diagnosed in two fetuses at 29 and 34 weeks. In the first case the fetus had a normal karyotype and no other structural heart defects, whereas in the second case there was a partial deletion of the long arm of chromosome 5 and an absent pulmonary valve syndrome. The association of absence of 'offsetting' with aneurysms of the membranous ventricular septum may represent spontaneous closure of ventricular septal defects initially extended to the inlet.

Allelic variations at the haploid TBX1 locus do not influence the cardiac phenotype in cases of 22q11 microdeletion

Microdeletion at the 22q11 locus is characterised by a high clinical variability. Congenital heart defects (CHD) are the most life-threatening manifestations of the syndrome and affect approximately 50% of patients carrying the deleted chromosome 22. The causes of this phenotype variability remain unknown although several hypotheses have been raised. It has been suggested that allelic variations at the haploid locus could modify the phenotypic expression. Regarding this hypothesis, TBX1 was thought to be a major candidate to the cardiac phenotype or its severity in patients carrying the 22q11 microdeletion. A mutational screening was performed in this gene, in a series of 39 deleted patients, with and without CHD. The results indicate that mutations in TBX1 are not likely to be involved in the cardiac phenotype observed in del22q11 patients.

Congenital heart disease and aneuploidy

Congenital heart disease (CHD) is one of the commonest prenatal diagnoses made on routine ultrasound screening. Overall, up to 33% of CHD are associated with fetal aneuploidy. However, some specific cardiac lesions have a significantly greater association with particular chromosomal abnormalities. The majority of fetuses with CHD and aneuploidy also have extra-cardiac anomalies and are best managed by a multidisciplinary team where the management and prognosis of the cardiac defect can be discussed in the context of the baby as a whole. It is therefore important for clinicians involved in the management of fetuses with CHD to be aware of the association of aneuploidy as well as the prognosis and management of these cases, so that they can appropriately counsel the parents. In this chapter, we review the frequency and types of aneuploidy associated with the commonly diagnosed CHD and discuss their management.

Unraveling the genetic and developmental mysteries of 22q11 deletion syndrome

Birth defects occur in nearly 5% of all live births and are the major cause of infant mortality and morbidity. Despite the recent progress in molecular and developmental biology, the underlying genetic etiology of most congenital anomalies remains unknown. Heterozygous deletion of the 22q11.2 locus results in the most common human genetic deletion syndrome, known as DiGeorge syndrome, and has served as an entry to understanding the basis for numerous congenital heart and craniofacial anomalies, among many other defects. Extensive human genetic analyses, mouse modeling and studies of developmental molecular cascades involved in 22q11 deletion syndrome are revealing complex networks of signaling and transcriptional events that are essential for normal embryonic development. Armed with this knowledge, we can now begin to consider the multiple genetic "hits" that might contribute to developmental anomalies, some of which could provide targets for in utero prevention of birth defects.

Association of tetralogy of Fallot with a distinct region of del22q11.2

Congenital heart defects (CHDs) appear in greater frequency among relatives of patients and in individuals with DiGeorge syndrome (DGS) or velo-cardio-facial syndrome (VCFS). A majority of these patients and part of the apparently nonsyndromic CHD patients with conotruncal defects manifest hemizygous deletions within chromosome 22q11.2 (del22q11). We tested myocardial tissues of 31 CHD patients, 21 with tetralogy of Fallot (TOF) and 10 with a double-chamber right ventricle (DCRV). DNA isolated from tissues removed at corrective surgery was analyzed for homo- or heterozygosity of nine polymorphic short tandem repeat (STR) markers along the 22q11.2 region. DNA from the blood of 45 healthy individuals represented the general population. Ten of the 21 TOF patients (48%) showed homozygosity for three or more consecutive markers, indicating deletions of various sizes. No such indication was found for DCRV patients. Heterozygosity for markers D22S1648, D22S941, and D22S944 was lower in the TOF group than in normal controls, defining a minimal critical region (MCR) for the deletion. Our findings support an association between TOF and hemizygosity in 22q11.2, suggesting a distinct region, between markers D22S1638 and COMT, that may harbor TOF susceptibility genes.

A common cis-acting sequence in the DiGeorge critical region regulates bi-directional transcription of UFD1L and CDC45L

Two to three megabase deletions on chromosome 22q11 are the cytogenetic findings most commonly associated with cardiac and craniofacial defects in humans. The constellation of clinical findings associated with these deletions is termed the 22q11 deletion syndrome. We had earlier described a patient with the 22q11 deletion phenotype who was hemizygous for an atypical 20 kb microdeletion in this region. The deletion included coding regions of two genes organized head-to-head, UFD1L and CDC45L, along with an 884 bp CpG-rich intervening region. Based on this genomic organization, we hypothesized that both genes may be co-expressed and co-regulated by sequences within this region. We demonstrate that expression of both genes is enhanced in a similar pattern in precursors of structures affected by the deletion. The intergenic region is sufficient to direct transcription most strongly in the developing pharyngeal arches and limb buds of transgenic mice and can also direct bi-directional transcriptional activation in a neural crest-derived cell line. Deletion analyses revealed that a 404 bp fragment closest to UFD1L is necessary and sufficient to direct this bi-directional transcriptional activity. These results reveal the presence of a conserved regulatory region in the 22q11 deletion locus that can direct simultaneous transcription of genes involved in ubiquitin mediated protein processing (UFD1L) and cell cycle control (CDC45L).

Requirement of CDC45 for postimplantation mouse development

CDC45 is required for the initiation of DNA replication in Saccharomyces cerevisiae and functions as a DNA polymerase alpha loading factor in Xenopus, but its role in mammalian DNA replication is unknown. To investigate the genetic and physiological functions of CDC45, we used a gene targeting strategy to generate mice lacking a functional CDC45 gene. Homozygous mutant mice lacking a functional CDC45 gene underwent uterine implantation and induced uterine decidualization but did not develop substantially thereafter. Detailed analysis of CDC45 null embryos cultured in vitro revealed impaired proliferation of the inner cell mass. These findings make CDC45 the only putative replication factor experimentally proven to be essential for mammalian development. The CDC45 gene localizes to human chromosome 22q11.2 in the DiGeorge syndrome critical region (DGCR). Almost 90% of individuals with congenital cardiac and craniofacial defects have a monoallelic deletion in the DGCR that includes CDC45. We report here that heterozygous mutant mice develop into adulthood without any apparent abnormalities, so that it is unlikely that hemizygosity of CDC45 alone is responsible for the cardiac and craniofacial defects in the congenital syndromes.

The mitochondrial thioredoxin system

Eukaryotic organisms from yeast to human possess a mitochondrial thioredoxin system composed of thioredoxin and thioredoxin reductase, similar to the cytosolic thioredoxin system that exists in the same cells. Yeast and mammalian mitochondrial thioredoxins are monomers of approximately 12 kDa and contain the typical conserved active site WCGPC. However, there are important differences between yeast and mammalian mitochondrial thioredoxin reductases that resemble the differences between their cytosolic counterparts. Mammalian mitochondrial thioredoxin reductase is a selenoprotein that forms a homodimer of 55 kDa/subunit; while yeast mitochondrial thioredoxin reductase is a homodimer of 37 kDa/subunit and does not contain selenocysteine. A function of the mitochondrial thioredoxin system is as electron donor for a mitochondrial peroxiredoxin, an enzyme that detoxifies the hydrogen peroxide generated by the mitochondrial metabolism. Experiments with yeast mutants lacking both the mitochondrial thioredoxin system as well as the mitochondrial peroxiredoxin system suggest an important role for mitochondrial thioredoxin, thioredoxin reductase, and peroxiredoxin in the protection against oxidative stress.

A requirement for neuropilin-1 in embryonic vessel formation

Neuropilin-1 is a membrane protein that is expressed in developing neurons and functions as a receptor or a component of the receptor complex for the class 3 semaphorins, which are inhibitory axon guidance signals. Targeted inactivation of the neuropilin-1 gene in mice induced disorganization of the pathway and projection of nerve fibers, suggesting that neuropilin-1 mediates semaphorin-elicited signals and regulates nerve fiber guidance in embryogenesis. Neuropilin-1 is also expressed in endothelial cells and shown to bind vascular endothelial growth factor (VEGF), a potent regulator for vasculogenesis and angiogenesis. However, the roles of neuropilin-1 in vascular formation have been unclear. This paper reported that the neuropilin-1 mutant mouse embryos exhibited various types of vascular defects, including impairment in neural vascularization, agenesis and transposition of great vessels, insufficient aorticopulmonary truncus (persistent truncus arteriosus), and disorganized and insufficient development of vascular networks in the yolk sac. The vascular defects induced by neuropilin-1 deficiency in mouse embryos suggest that neuropilin-1 plays roles in embryonic vessel formation, as well as nerve fiber guidance.

Congenital heart disease in mice deficient for the DiGeorge syndrome region

The heterozygous chromosome deletion within the band 22q11 (del22q11) is an important cause of congenital cardiovascular defects. It is the genetic basis of DiGeorge syndrome and causes the most common deletion syndrome in humans. Because the deleted region is largely conserved in the mouse, we were able to engineer a chromosome deletion (Df1) spanning a segment of the murine region homologous to the human deleted region. Here we describe heterozygously deleted (Df1/+) mice with cardiovascular abnormalities of the same type as those associated with del22q11; we have traced the embryological origin of these abnormalities to defective development of the fourth pharyngeal arch arteries. Genetic complementation of the deletion using a chromosome duplicated for the Df1 DNA segment corrects the heart defects, indicating that they are caused by reduced dosage of genes located within Df1. The Df1/+ mouse model reveals the pathogenic basis of the most clinically severe aspect of DiGeorge syndrome and uncovers a new mechanism leading to aortic arch abnormalities. These mutants represent a mouse model of a human deletion syndrome generated by chromosome engineering.

UFD1L and CDC45L: a role in DiGeorge syndrome and related phenotypes?

Molecular genetics is contributing to the understanding of normal and abnormal cardiovascular development and morphogenesis. Deletions of chromosome 22q11.2 have been associated with distinct phenotypes that result from a failure to form derivatives of third and fourth branchial arches, including DiGeorge syndrome (DGS) and velo-cardio-facial syndrome (VCFS). The biochemical mechanisms underlying these phenotypes remain undetermined. A recent study provides new insight into the mechanism by which gene deletions produce the DGS and VCFS phenotypes.